South Korea and the United States signed a civilian nuclear energy cooperation agreement Monday, bringing a formal end to years of tough negotiations that had required the pushing back of an initial deadline.

South Korean Foreign Minister Yun Byung-se and U.S. Energy Secretary Ernest Moniz signed the revision to the 1974 nuclear cooperation pact that mainly stipulates South Korea's obligations in exchange for receiving technical support and other nuclear cooperation from the U.S.

In April, the two sides initialed the deal after winding up more than four years of negotiations that centered on reconciling Seoul's demand for the right to reprocess spent nuclear fuel and enrich uranium with Washington's concerns about proliferation.

The new deal still bans Seoul from reprocessing and enrichment, but it opens the way for the Asian ally to begin research into a new technology for spent nuclear fuel recycling, known as "pyroprocessing," and to make low-level enriched uranium with U.S. consent.

Both Yun and Moniz called Monday's signing a "historic moment."

"We feel that this new agreement will upgrade our relations to a new height and -- thanks to this innovative, very much future-oriented agreement -- we will be able to work together in many new areas," Yun said during the ceremony at the Energy Department.

"This will get us securing a stable supply of nuclear fuel and manage spent fuel more efficiently and there are many areas where we can collaborate together in third countries. So this will give a big boost to Korean and American businesses as well," he said.

Moniz emphasized South Korea and the U.S. are strong advocates for nonproliferation objectives.

"This agreement solidifies that again as we enter a new phase of our cooperation," he said. The secretary also said that the U.S.-Korea partnership is an "absolute anchor" for peace and stability in the region and the partnership cannot be stronger.

Since the initialing, the two governments conducted legal and other internal reviews before South Korean President Park Geun-hye approved the deal in a Cabinet meeting on June 9 and U.S. President Barack Obama gave it a go-ahead last week.

The two countries had originally planned to hold a signing ceremony during Park's visit to Washington that had been set for this week, but the plan fell through as she postponed the trip due to the massive outbreak of Middle East Respiratory Syndrome (MERS) in South Korea.

Now that the agreement has been formally signed, the U.S. Congress will review the agreement for 90 congressional days, which could take up to six months, and the pact will go into effect if no opposition is raised during the process.

Widespread views are that no objections will be raised.

No legislative approval is necessary in South Korea, officials said.

The two countries began negotiations in 2010 to renew the agreement that had originally been scheduled to expire in 2014. After failing to find a compromise, the two sides extended the existing agreement by two years to buy time for more negotiations.

Meanwhile, Yun was scheduled to meet with U.S. National Security Adviser Susan Rice later in the day for talks that are expected to include rescheduling Park's visit to Washington, the North Korean nuclear standoff and other pending issues.

"We will push for a U.S. visit within the earliest possible time," Yun told reporters after the signing ceremony. "We can say it will be within this year for us."

Asked about North Korea's expression of willingness to hold government-level talks with the South, Yun said that the door for dialogue with Pyongyang is always open and the Seoul government will deal with the issue with an open-minded attitude. (Yonhap)